The invention is also concerned with a method for the manufacture of such a humidity sensor.
FIGS. 1 and 2 show the cross-sections of two capacitive humidity sensors of types known in the prior art. In the construction in accordance with FIG. 1, there are two bottom electrodes 2 between the substrate 1 and the isolation layer 15. In addition to the parts of the above construction, the construction of FIG. 2 also includes a surface electrode 3 placed on top of the isolation layer 15.
As a rule, the electrode structure of a capacitive humidity sensor must meet, e.g., the following requirements:
(a) humidity must have unhindered access of penetration into the isolation material, PA1 (b) the electrodes must be electrically well conductive and mechanically durable, and PA1 (c) the electric field between the electrodes must not penetrate onto the sensor surface, where there may be electrically conductive impurities.
The construction of FIG. 1 meets the requirements a and b, but not the requirement c. The construction of FIG. 2, on the other hand, meets the requirement c, but the requirements a and b substantially exclude each other.
As regards the state of prior-art technology, reference should be made in particular to the following publications:
[1] K. E. Bean, "Anisotropic Etching of Silicon" IEEE Transactions on Electron Devices UED-25 (1978) No. 10, pp. 1185-93 PA0 [2] U.S. Pat. No. 3,397,278 (Pomerantz) PA0 [3] FI Pat. No. 48,229 (Suntola).